Incandescent lamp, filament therefor and process and apparatus for making the same



Oct. 22, 1935. s. RUBEN 2,

* INCANDESCBNT LAIP, FILAIBNT THEREFOR AND PROCESS AND APPARATUS FORIAKING THE SAIE Filed Sept. 23, 1929 I NVENTOR JAM/5L FI/BEA/ PatentedOct. 22, 1935 PATENT OFFICE INCANDESCENT LAMP, FILAMENT THERE- FOR ANDPROCESS AND APPARATUS FOR MAKING THE SAME Samuel Ruben, New York, N. Y.,mignor to Sirian Lamp Company, Newark, N. J., a corporation of DelawareApplication September 23, 1929, Serial No. 394,849

2 Claims.

This invention relates to incandescent lamps, filaments therefor andprocess and apparatus for making the same. More specifically, theinvention relates to a new type of electric lamp suitable for use inhouse lighting, automobile lighting, projector illumination and variousother types of illuminating devices. The invention also pertains to a.coated filament adapted to be heated by the passage of electricitytherethrough which is an improvement over the ordinary simple form oftungsten filament.

Among the objects of the invention is the provision of a filamentadapted for use in lamps and other means of light projection andillumination, having a. core or base of refractory material such astungsten on which is placed a coating having a material rich in lightemitting characteristics, whereby a relatively high luminous efiiciencyis obtained.

Another object is to provide a filament in an electric lamp whichconsists of a core and a coating, the coating having bonding means toeffectively unite the material of the coating to the core so that thereis minimum loss of heat efiiciency intermediate the core and the coatingwhen the filament is utilized as an illuminant for electric lamps.

Still another object of the invention is to provide a bonding agentbetween a highly luminous coating and a refractory filamentary corewhich has high melting'point characteristics similar to that of thematerial of the coating and core.

Another object involves the formation of an incandescent lamp of thegas-filled type, wherein pressure requisite to diminish the vaporizationof the filamentary material is'low relative to that of the ordinarygas-filled lamp.

Additional objects of the invention are the provision of a lamp whichwill operate with eificiency greater than that of the tungsten lamp andto provide a process and apparatus for manufacturing the improvedfilament and lamp.

Other objects will appear from the detailed description following, andfrom the drawing accompanying the description, in which Fig. 1 is a viewof an ordinary type of automobile lamp provided with a filament of theimproved type herein set forth; and

Fig. 2 is an enlarged fragmentary view of a section of the lampfilament.

The use of tungsten and similar refractory filamentary substances asfilaments for electric lamps is well known. Attempts to improve ontungsten for electric lamps have been. made by many inventors, not onlyby substitution of the material but by the addition of other elements orcompounds either as a part of the tungsten material or as a coating. Theresults of most of these investigations and researches, however, havebeen negative to the extent that the eificiency increases, if any, weretoo small to have practical value, especially as compared with therigidity and refractory characteristics and high illuminating ability oftungsten.

Certain compounds have been known such as the material of the Welsbachburner and the Nernst glower which have a relatively high lightradiation in the luminous range of temperatures, and efiorts have beenmade to combine these and similar materials as a coating to a refractorycore such as tungsten and molybdenum. None of these attempts, however,have achieved note worthy success and my searches have indicated thatthe cause might be attributed to loss of heat energy between the coatingand the core so that the effect of the coated filament was undesirable,the filament having a lesser efliciency thanif tungsten alone were used.

I have succeeded in overcoming the difilculties above mentioned in thecoated filament by utilizing a highly refractory material as a coatingwhich does not have a tendency to become a good conductor when heated,and also by developing a bonding agent which secures a very intimate andpermanent contact between the core and the coating, thus effectivelydiminishing the heat gradient between-the coating and the core, andincreasing the lamp efilciency.

As the core material, tungsten is preferably used, although molybdenumor other refractory metals or substances may be employed. Thisfilamentary core is coated by a mixture of thorium oxide or thoria andcerium oxide or ceria, bound together into the core by means of boronoxide. The thoria-ceria. mixture is made by heating a water solution ofcommercial thorium nitrate and cerium nitrate, the proportion being byweight 99% of thorium nitrate and 1% of cerium nitrate. This material isheated so as to be completely evaporated and then to the decompositiontemperature, thus leaving a mixture of thoria and ceria, the proportionby weight being approximately 99% thoria and 1% ceria. To this mixtureis added 25% by weight of crystallized commercial boric acid of variouscompositions (such for example, as 3H2O, B203 or 11330: or B(OH) 3 orH2B4O7) and enough water to form a thin paste. The entire mixture isthen ground in a ball mill of an ordinary type, preferably for a periodof about twenty (20) hours, so as to give an intimate and finely grainedmixture, which is then in condition appropriate for coating on thefilament.

As utilized for incandescent electric lamps, the coating mixture may nowhe applied directly to the formed filaments. Prior to the coatingoperation, however, the filaments should be thoroughly cleaned with asolution made of caustic soda and the coating may then be applied byappropriate means as a thin layer on the filament. The coating asapplied has sufiicient consistency so that it adheres to the filamentuntil it is properly secured to its base inthe lamp and then placed onthe pump racks and subjected to exhaustion by vacuum pumps. When theproper degree oi exhaustion has been obtained, the lamp filaments areslowly heated by electric current until the gas and vapor of thefilament and base have been removed and the material 01' the filamentcoating sinters. After sintering, the lamps are available forilluminating purposes and the filament may be brought to an incandescenttemperature; but in the form of lamp herein described, in order toprolong the life or the lamp by decreasing the vaporization rate oi thefilament, gas is introduced into the lamp bulb-after the sinteringoperation at low pressures in a range not exceeding 75 millimeters, andpreferably in the neighborhood of 25 millimeters. Argon gas isdesirableior this purpose, although nitrogen is also available and maybe used with good results. Other gases having a low heat conductivityand inert chemical characteristics may also be used.

Referring to the drawing, there is indicated in Fig. i an ordinary typeof automobile lamp for headlamps having an envelope in, base H, stem l2,filament supports 03, and a coiled filament I. In Fig. 2, the enlargeddetall'shows a wire core l5 of tungsten or similar material, on which isaffixed the coating I 6 consisting of a mixture of boron oxide, thoriaand ceria.

Both thoria and ceria have high melting points and when combined as amixture in the proportions stated, have high selective luminosity whenheated to the luminous range of temperatures. When used alone, however,and with other types of binders, this mixture becomes a fairly goodconductor at high temperatures, thus bringing about undesirable currentand electrolytic cf i'ects. Boric acid used as a binder in the presentinvention is converted to boric oxide in the neighborhood of 1000 C.which also at that temperature is plastic and thoroughly binds theradiant layer to the tungsten base, giving an integralstructure with anegligible temperature gradient between the base and radiator. Moreover,boric oxide is an excellent resistant material at high temperatures and,therefore, will not conduct any appreciable quantities of current sothat the deficiencies above noted with reference to thoria and ceriaalone are nullified by the intimate lntermixture of the poorlyconducting boric oxide.

By the process as herein above described, applicant has developed afilament which is highly refractory, which has a surfacing ofselectively luminous radiating material and which material is intimatelybound to the refractory base material so that heat losses between thesurfacing or coating and the base are negligible. By this means, whenemployed in the bulb containing argon, for example, under low pressures,an electric lamp is provided which has an efliciency exceeding that oi alamp with a tungsten filament as measured in watts per candle power.

Various modifications may, of course, be made of the exact process andproducts described, and therefore the terms oi the claims heretoappended are to be interpreted in their broadest sense.

Having thus described my invention, what I claim is:

l. A filament for an electric lamp comprising.

a refractory metal wire and a coating thereon formed or a mixture 01'approximately 99% of thorium oxide and 1% of cerium oxide through whichboric acid is uniformly distributed to the extent of 25% of the thoriumand cerium oxide mixture.

2. A filament for an electric lamp comprising a refractory metal wireand a coating thereon formed of a mixture of approximately 99% orthorium oxide and 1% of cerium oxide through which boron oxide isuniformly distributed to an extent less than 25% by weight 01 themixture of thorium and cerium oxide.

SAMUEL RUBEN.

